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C#, 201 bytes, by milk

using System.Linq;
_=>string./**/
#if trueJoin
(/*"*//*/""/**//*/"*/",
#else
Concat(//*/,
#endif
from m in" "let p=$@"{@" 
"[1]}"from i in(_.Replace(p,@""))where!new[]{p[0]^32^10}.Contains(i)select@i);

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